In 1996, the Hubble Space Telescope took a set of long-exposure images of a single tiny patch of the sky. This Hubble Deep Field (HDF) survey obtained data on over 1,500 galaxies stretching back to the earliest days of the Universe, in a wide range of wavelengths of light. The HDF has provided riches for astronomers but also an enduring mystery in the form of HDF 805.1—the brightest object in the infrared part of the spectrum. The wavelength in which HDF 805.1 is brightest corresponds to dust surrounding strong star formation, but the object is invisible in other types of light. Without other data, astronomers couldn't determine the size of, or distance to, HDF 805.1.

Now a research team using the Plateau de Bure Interferometer (PdBI) in the French Alps and the Jansky Very Large Array (VLA) in New Mexico have detected HDF 805.1 in submillimeter radio light. As Fabian Walter et al. report in Nature, the location of HDF 805.1 corresponds to a bright source about 12.6 billion light years away, meaning the object formed only 1.1 billion years after the Big Bang. The light measured from the HDF and PdBI together suggests a galaxy about 130 billion times the mass of the Sun (comparable in mass to the Milky Way) with a high rate of star formation. This discovery is surprising for two reasons: the star formation rate is higher than predicted for a galaxy that early in the Universe's history, and the amount of dust required to hide it completely in visible light is larger than expected.

HDF 805.1 is one of a class of objects known as submillimeter galaxies (SMGs), not because they are tiny, but because they emit light primarily at submillimeter wavelengths. By the spectrum of light they emit, we know SMGs produce new stars at a rapid rate. But the environment of the star formation is very dusty, blocking almost all the visible light. Without that spectral information, it is difficult to measure their distance, as well as the size of the galaxy. However, SMGs are potentially valuable in mapping the history of galaxies and star formation in the Universe, so astronomers have tried since 1996 to solve the riddle of HDF 805.1.

New techniques, highlighted in the present study, use dedicated submillimeter instruments to measure the spectra of the SMGs. In turn, this reveals their redshift—the amount the wavelength of light is stretched as the Universe expands. Redshift is a proxy for distance and age: a larger redshift indicates the light was emitted earlier in time and traveled farther to reach us. For HDF 805.1, the researchers focused on two carbon monoxide (CO) and ionized carbon (CII) spectral lines, which are easily identifiable signatures in star-forming regions.

Both types of observations agreed: HDF 805.1 is at a high redshift, meaning the wavelength observed at Earth is stretched 6.1 times its emitted value. Using the Hubble relation, this redshift means the galaxy emitted the light only 1.1 billion years after the Big Bang. While this isn't the earliest galaxy ever seen, HDF 805.1 certainly has more active star formation than its peers.

Additionally, dust in galaxies is comprised of atoms like carbon and oxygen, (formed by stars) rather than being present in the very early Universe. The amount of dust present in HDF 805.1 must be sufficient to hide it in visible-light wavelengths, but it's uncertain where it all came from that early in the Universe's history. Finding similar high-redshift SMGs may help resolve that problem. Currently, only a handful of similar star-forming galaxies (that is, submillimeter-bright galaxies with no evident supermassive black hole) are known, and none are as distant.

As is often the case, solving one mystery—the distance and character of HDF 805.1—has led to further questions. However, the advent of new submillimeter telescopes, including the Atacama Large Millimeter Array (ALMA) in Chile, should help enlighten us about the history of galaxies and star formation in the early Universe.